I'd really like to see a 512 that gets 20mpg. It will never happen. Tom's 422 does well at 17mgh... and that's with a big carb....lol. Then you have bellhousing, flywheel, mounts, radiator, brackets, etc. BB is way more $$, for no milage bonus. I could get a 440 to do 20 on the highway... built stock but with modern machining and careful tuning. Butit would be down on power because of the milage factor. I stick with 360 for budget, fitment, and balance of performance.